Microstructure Evolution of Laser Welded 301LN and AISI 304 Austenitic Stainless Steel

Cold-rolled plates of metastable austenitic stainless steel (SS) 301LN are the main materials for manufacturing lightweight railway passenger cars, where non-penetration laser welding is used to assemble weld-free sidewalls to improve corrosion resistance and joint strength. Since fast solidified austenitic SSs are susceptible to thermal cracking, microstructure evolutions of laser welded 301LN and AISI 304 were compared and simulated thermal analysis and composition-partitioned detection were carried out. The results showed that both laser welds of 301LN and AISI 304 presented microstructures solidified by mixed modes. Most areas of the 301LN weld showed microstructures solidified with primary δ -ferrite, while the weld root showed a microstructure solidified with primary γ -austenite due to the faster-rate cooling. Most areas of the AISI 304 weld showed microstructures solidified with primary γ -austenite, while the last solidified top central area presented a microstructure solidified with primary δ -ferrite due to the higher local temperature and slower-rate cooling. In addition, the concentration of nickel content caused by macro-segregation led to the 301LN weld center solidify as primary γ -austenite. The microstructure indicated that the laser weld bead of 301LN had higher thermal cracking resistance than that of AISI 304.